Large diameter steel pipe is formed by bending a flat plate of low carbon steel of the desired thickness into a tube with the longitudinally extending edges abutting. A windrow of granular flux is deposited on these edges of the first side to be welded and one or more electrically energized mild steel or low alloy steel electrode(s) is advanced through the windrow to establish an arc between the end of the electrode(s) and the longitudinally extending edges. The electrode(s) is then moved along the edges. The arc melts both the edges and the end of the electrode(s) to form a molten weld puddle which solidifies as the electrode(s) moves on. At the same time, the arc melts some of the granular flux which floats on top of the molten weld puddle and solidifies after the molten steel in the weld pool puddle solidifies, thus protecting the weld puddle from the atmosphere and shaping the surface of the solidified weld bead and giving it an appropriate surface texture. The unmelted flux is then recovered and can be reused. The melted flux, now solidified as a slag, is then removed and usually reprocessed. The edges are then welded from the reverse side with the bottom of the weld overlapping or intersecting with the weld from the opposite side. The pipe is then usually stressed by hydraulic pressure on the inside sufficient to stress the metals beyond the yield point and make the pipe round. The requirements of a flux for pipe fabrication are deep penetration, so that the weld can be accomplished in two passes (one inner and one outer), a low weld bead profile so that no or a minimum of weld metal must be removed to give a smooth pipe contour and no undercutting at the edges.
The fluxes used are mixtures of various known fluxing ingredients such as the fluorides selected from the group of calcium, sodium, potassium, barium, magnesium, strontium and lithium, the oxides selected from the group of aluminum, magnesium, silicon, strontium, titanium, calcium, zirconium, manganese, and potassium, and the like and sometimes deoxidizers, all in carefully controlled proportions selected to give: a desired solidifying temperature to the molten slag; desired slag removal characteristics; desired welding characteristics; and, most importantly, desired mechanical properties to the deposited weld bead.
The various flux ingredients, in powdered form, are thoroughly intermixed and then either: fused by heating to a temperature where all of the ingredients are melted so as to react with one another, allowed to cool, and, finally crushed to a desired granule size; or, agglomerated by including a low temperature binder, such as sodium or potassium silicate, in the mixture and then heating the mixture to a temperature sufficient to dry the binder and bind the other ingredients in unreacted state into granules of the desired size.
Fused fluxes, assuming the same ingredients, are more expensive to manufacture than agglomerated fluxes because of: the greater energy requirements to melt all of the ingredients; the cost of sophisticated equipment to withstand the higher temperatures; and, the additional steps of cooling the liquid mass to a solid and then crushing it to the desired granule size.
Agglomerated fluxes require the use of a relatively inexpensive binder. With the lower firing temperatures and the fact that the ingredients are never in a liquid state, they inherently form into granules of the desired size in the manufacturing process which normally employs a rotating heated kiln.
Agglomerated fluxes are easier for the operator to handle. They are granular and can be handled easily with no discomfort. Fused fluxes may be likened to broken glass with the granules sometimes being sliver like or having sharp edges.
Agglomerated fluxes do not melt as easily as fused fluxes so that less agglomerated flux is consumed when welding. This decreases the cost of producing a weld which is very desirable.
Agglomerated fluxes also allow greater flexibility in adding alloying metals to the formulation because at the high temperatures necessary to melt fused fluxes, such metals are consumed. Alternatively, the metals in powder form may be added to the crushed granules but problems result here because during shipment, the powdered metals tend to settle out of fused fluxes. In agglomerated fluxes, the powdered metals may be mixed with the ingredients before heating and are evenly distributed throughout the agglomerated granules.
Agglomerated fluxes produce a deposition rate which can be 25% higher than similar fused fluxes at the same welding current. This results in higher productivity which is an advantage in most applications where increased deposition rates reduce the welding time required to fill a given joint volume or to produce the required weld size. In pipe manufacture, however, a high deposition rate is not desirable because it results in excess weld reinforcement and a poor weld profile. For this application it is desirable to produce a low weld profile with a minimal change in the contour of the surface of the welded pipe.
Prior to the present invention, agglomerated fluxes had an additional disadvantage in pipe welding when compared to the fused fluxes, namely, for a given weld current the penetration of the weld into the base metal was less than that produced by comparable fused fluxes. High penetration is necessary to ensure that after the inner and outer weld passes are completed, the weld extends across the entire thickness of the abutting edges to ensure that the weld seam will not be weaker than the rest of the pipe. This is particularly important because of the exceedingly high stresses imposed on the weld bead during the hydraulic forming step. However, if the welding current were increased to increase the penetration, then the deposition rate became so high as to produce a weld bead with a high profile. High penetration is also desirable because it provides sufficient overlapping of the weld beads made from each side of the pipe to ensure tie-in without requiring critical tolerances for the alignment of the welding electrodes with the butted seam.
Heretofore, despite the cost and other advantages of an agglomerated flux above described, operators welding the longitudinally extended edges of a formed pipe have preferred the fused fluxes because of the high penetration, low bead profile characteristics obtainable with such fluxes.